Immersion lithography is a technique whereby a liquid of high refractive index and high transparency fills the air gap between the optical system's projection lens and the silicon wafer target, thereby enabling finer feature sizes to be printed onto the photoresist-coated silicon wafer.
Water, with a refractive index of 1.44, is in commercial use as an immersion liquid. One approach to obtaining higher refractive index is to combine water with other substances, as described, for example, in Zhang et al., U.S. Published Patent Application 2005/0173682.
Wheland et al., U.S. Published Patent Application 2005/119371, discloses use in immersion lithography of high transparency alkanes having refractive indices in the range of 1.5-1.7.
Deng et al., U.S. Published Patent Application 2005/0164502, discloses use of siloxane polymers and oligomers as immersion medium in immersion lithography.
Taku et al., WO2005/117074, discloses siloxane liquids for use as immersion liquids having refractive index of ca. 1.55 at 193 nm, and represented by the formula:
where R is a hydrocarbon or halogenated hydrocarbon radical having 1-8 carbons, and n ranges from 0-40 but is preferably 0.
Schott et al., Z. anorg. allge. Chem, 459, pp. 177-186 (1979) discloses dialkoxy and trialkoxy compounds.
The only materials than have been identified to date that fulfill the need for refractive index at 193 nm of greater than 1.6 and high transparency are some saturated cyclic hydrocarbons; e.g. cyclooctane, bicyclohexyl, decahydronaphthalene, and the like. Although they have high index, these materials appear to have an upper index limit of approximately 1.65 at 193 nm. Moreover, the range of available or easy to synthesize structures is limited. Thus, there remains a need for easy to synthesize compositions that provide higher index of refraction at 193 nm and high transparency.